A current shunt may be used to measure current flow in a circuit. The current to be measured may flow through a series-connected current shunt of known resistance, and the voltage across the shunt may be measured. Utilizing Ohm's Law (I=V/R), one may calculate the current based upon the measured voltage and the known resistance.
A battery system including a battery management system (BMS) in combination with a battery may utilize the current shunt to determine the current flowing in to, and out of, the battery. The BMS may use this information for such purposes such as to determine such criteria as the charge level, or state of charge, of the battery.
Particularly in the case of a system with high currents, it is desirable to minimize the resistance of the shunt as low as possible in order to minimize energy loss in the system. For example a resistance below 70 μΩ's (micro-Ohms) might be acceptable for a system expected to have 1000 Amps of current flow, as this would result in a voltage of 70 μΩ's times 1000 Amps, which equals 0.07 Volts. Power loss associated with this would be 1000 Amps times 0.07 Volts, which equals 70 Watts. Lower power loss would of course be better, but a minimum amount of resistance is necessary to provide a usefully measurable voltage.
Current shunts may be made of a conductive material such as a conductive metal or metal alloy. The resistance of many conductive materials may vary with temperature. For the current calculation to be sufficiently accurate over a range of temperatures, the resistance of the conductive material should be relatively constant over the range of temperatures.
One such conductive material having a relatively constant temperature coefficient of resistance, and which has been used in current shunts, is a copper-nickel alloy known as “constantan.” Constantan typically consists of 55% copper and 45% nickel. However constantan may be a relatively expensive material. And the expense of constantan may become progressively more significant in high current applications, as progressively more constantan may be required to carry the high current.